This invention relates to an improved gas shroud apparatus for protecting a molten pouring stream issuing from a bottom pour vessel from atmospheric reoxidation. The invention is particularly well suited to the casting of steel from a tundish.
In the continuous casting of molten metals such as steel the molten metal is teemed from a ladle into an intermediate pouring vessel known as a tundish. Each tundish has at least one and normally a multiplicity of pouring nozzles in its bottom wall to feed a like number of continuous casting molds. The molten metal pouring stream is subject to atmospheric reoxidation which will result in the formation of oxide inclusions in the continuous cast product. Additional production cost is required to remove such inclusions prior to fabricating the cast product into a finished product. In addition, trapped inclusions result in unacceptable product cleanliness.
Shrouding of molten metal pouring streams in the continuous casting of steel is well known in the art as exemplified by Lyman U.S. Pat. No. 3,572,422, Pollard U.S. Pat. No. 3,908,734, Holmes U.S. Pat. No. 3,439,735, Newhall et al U.S. Pat. No. 3,616,843 and Coward et al U.S. Pat. No. 4,084,799.
In order that stream characteristics and the metal in the continuous casting mold can be observed, there must be a gap between the bottom of the shroud and the metal meniscus in the mold. In addition, it is desirable to have a readily removable shroud to accommodate other apparatus beneath the pouring stream such as a launder to divert the pouring stream from the mold when there is a breakout or other problem. In addition, the shroud must be removable to allow access to the pouring nozzle by an oxygen lance or chill plug.
As has been seen in Coward et al U.S. Pat. No. 4,084,799, stream cleanliness is highest when the shroud is tightly sealed against the tundish to prevent entrance of atmospheric oxygen to the top of the shroud and entrainment of oxygen in the pouring stream. Previous shrouds which were tightly positioned against the bottom of the tundish have been supported by a mechanism either on the pouring floor as taught by Coward et al, or on the mold itself as in the bellows-type shroud of Newhall et al. We have invented a shroud apparatus which is pivotally suspended from a bottom pour tundish or other bottom pour vessel and is held sufficiently tight against the bottom of the tundish to provide a substantially gas tight seal by the expedient of providing a counterweight at the outer extremity of the shroud support arm. The counterweight may be adjustably positionable on the support arm if desired. The shroud may readily be moved into and out of the operative position as required with a minimum of effort. The shroud of the present application is particularly well suited to installations where a launder is employed as well as to installations not employing a launder. The shroud may be completely removed by lifting it from its suspension means, or it may be pivoted out of the way by striking it with a launder, or it may simply be rotated out of the operative position manually.
Shrouding with liquid nitrogen is currently practiced at some continuous steel casting installations. The liquid nitrogen is sprayed onto the tundish pouring stream from an annular spray ring which encircles the pouring stream. This is an efficient but extremely costly means of shrouding which uses copious quantities of nitrogen. The present invention is extremely inexpensive and uses nitrogen sparsely. In fact, normal nitrogen usage when casting 51/8".times.51/8" billets is less than 85 cu. ft. per ton of product.